This invention relates to coextrusion, particularly of a laminate having multiple barrier layers.
As illustrated by U.S. Pat. Nos. 4,197,069 and 4,533,308 to Cloeren, an extrusion apparatus having a vane blade between flow channels, is known. The vane blade provides for stream convergence at substantially equal flow velocities. As exemplified by U.S. Pat. No. 4,780,258 to the inventor, also known is an extrusion apparatus and process useful for coextruding a laminate containing more than a single barrier layer.
In the packaging industry, a layered sheet or film having a single barrier layer of, for example, ethylene vinyl alcohol polymer is widely used. This copolymer is commonly known as EVOH. A commercially available EVOH copolymer contains about 40 mole per cent ethylene and about 60 mole percent vinyl alcohol.
A barrier layer serves as a barrier to gases such as oxygen. In the food packaging industry, a barrier layer prolongs freshness and shelf-life, and retains aroma. However, imperfections in a barrier layer will interrupt the integrity of the barrier.
In using coextruded sheet, as for instance in forming containers, considerable scrap is produced. For economic reasons, it is desirable to recycle the scrap. As the percentage of scrap within a coextruded sheet increases, production of the sheet becomes relatively more cost-effective. It is therefore highly advantageous to be able to include within a sheet a high percentage of scrap, say about 30-65%, of the total volume.
Difficulty has been encountered in using scrap containing EVOH and polypropylene, a commonly used skin layer material, which is a good barrier to moisture.
When EVOH and polypropylene are used as the barrier and skin layer materials, respectively, of a coextruded sheet, and when the barrier material stream is disposed between a stream providing a high percentage of regrind, that is, scrap, to the resultant sheet, and the skin layer material stream, an interfacial instability in the form of an undesirable wavy pattern may be observed at the barrier material interface closer to the skin layer material stream.
This pattern may be eliminated by dividing the regrind stream into portions and disposing one portion next to the polypropylene stream. Using the regrind as a wall-contacting stream within an extrusion apparatus has been found to be additionally beneficial. Nevertheless, a surface flaw known as the applesauce or sandpaper effect, may be observed in the coextruded sheet.
Furthermore, organoleptic contamination by the regrind should be prevented, particularly when the coextruded sheet is used as a food packaging material.
Therefore, there is a need for an improved coextrusion process that is able to provide a coextruded laminate having a barrier layer and containing a high percentage of regrind, that is, at least about 30 to 35% of the total volume. Advantageously, such an improved coextrusion process would overcome the interfacial instability associated with an EVOH barrier layer and the applesauce effect. Beneficially, such an improved coextrusion process would also prevent organoleptic contamination by the regrind.